Project Summary: The long-term objective of this project is to determine how the high molecular weight isoforms of Fibroblast growth factor 2 (HMWFGF2) contribute to osteoarthropathy. Osteoarthritis (OA) is the most common form of arthritis and has a major negative impact on human health and cost to the health care system. Degenerative osteoarthropathy, a form of OA is prevalent in young individuals with X-linked hypophosphatemia (XLH) and common to all older XLH individuals and is a major cause of morbidity in these subjects. HMWTg mice phenocopy the Hyp mouse homolog of XLH that develops severe OA. We also previously published that HMWFGF2 isoforms are overexpressed in osteoblasts and osteocytes of Hyp mice. Of relevance we recently published that similar to Hyp mice, HMWTg mice develop progressively severe OA with age. In contrast mice overexpressing low molecular weight FGF2 isoform (LMWTg) in osteoblast lineage cells do not develop OA. Our preliminary data also show that mice in which the FGF2HMW isoforms are selectively ablated do not develop OA compared with aged WT littermates. Of mechanistic relevance our preliminary studies show upregulation of FGF23/FGFR1/MAPK and BMP/Smad1/5/8 in HMWTg knee joints but not LMWTg joints. We therefore wish to explore the potential role of FGF23 in HMWTg induced OA since we published that HMW but not LMW FGF2 transcriptionally regulates FGF23. Furthermore, FGF23 in OA is an understudied area thus studies of FGF2 isoforms modulating FGF23 in OA could also impact the field. We provide novel preliminary data that in vivo administration of FGF23 neutralizing antibody ameliorates the OA phenotype in HMWTg mice. These findings support our overall hypothesis that HMWFGF2 isoforms contribute to OA via FGF23/FGFR1/MAPK signaling. We also wish to investigate the mechanism of joint homeostasis in LMWTg and HMWKO mice since LMWFGF2 appears to protect the joint from OA development. Our aims are: Aim 1a:Define the role of FGF23/FGFReceptor signaling in HMWFGF2 induces osteoarthropathy. Aim 1b: Determine the functional effects of overexpressing of low molecular weight FGF2 or selective ablation of HMWFGF2 isoforms to confer resistance against osteoarthropathy progression in mice. Aim 2: Define the mechanism(s) by which HMWFGF2 induces osteoarthropathy. Our preliminary in vitro studies suggest differential activation of FGF23/FGF receptors and downstream signaling pathways for the catabolic effects of HMWFGF2 versus anabolic effect of LMWFGF2 in joint homeostasis. Murine chondrogenic cell line ATDC5 transduced with FGF2HMW and FGF2LMW cDNAs and primary murine articular chondrocytes obtained from Vector, HMWTg and LMWTg mice will be used to interrogate intracellular signaling pathways that are unique for HMW or LMW in developing or preventing development of OA. Since the role of FGF2 in OA is controversial with studies supporting both catabolic and anabolic effects of FGF2 and since the potential role of FGF2 isoforms in OA has not been investigated, our studies could significantly impact the field.